The microtubules are essential elements for numerous functions in all sorts of cells. In plants in particular, they play a central role in several important phenomena, in particular those relating to morphogenesis. In all eukaryotes, the microtubules are composed of a large number of highly conserved proteins, the most abundant being the tubulins. Two principle subunits of proteins have been described, which are called .alpha.-tubulin and .beta.-tubulin. The essential functions of the microtubules and their ubiquitous distribution have often suggested the idea that the genes encoding tubulins are a good example of highly and structurally expressed genes. In fact, the subunits of tubulin are encoded in eukaryotes by a family of genes. In the plant domain, the .alpha.- and .beta.-tubulins have been studied in some plants such as maize, Arabidopsis, soya bean, peas and carrot. In all these cases, the respective genomes encode multiple .alpha.- and .beta.-tubulin genes which are differently expressed during the development of the plant. In Arabidopsis, careful analysis of a family of tubulin genes revealed 15 genes (6 .alpha.-tubulins and 9 .beta.-tubulins) encoding these proteins (Kopczak et al., 1992; Snustad et al., 1992). In maize, 3 .alpha.-tubulin genes have been cloned and sequenced (Montoliu et al., 1989; Montoliu et al., 1990), and six others can be detected by PCR analysis of the genomic DNA (Montoliu et al., 1992). According to a recent study, at least six DNA sequences of distinct .alpha.-tubulins were cloned and characterized from maize tissues (Villemur et al., 1992).
The .alpha.-Tub 1 and .alpha.-Tub 2 genes derived from maize are arranged in tandem, separated by at least 2 kilobase (kb) pairs of DNA. They are expressed in all maize meristematic tissues with high levels of expression in the radicular system (Montoliu et al., 1989). The .alpha.-Tub 1 gene is expressed in all the analyzed tissues at a higher level than the .alpha.-Tub 2 gene and is, in addition, highly expressed in the pollen (Montoliu et al., 1990). Hybridization experiments in vitro show that inside meristematic tissues, the .alpha.-Tub 1 gene is expressed during the quiescent central activation (Rigau et al. in Press). In Arabidopsis, a gene was found to be specifically expressed in pollen but none showed the same pattern of expression as the maize genes .alpha.-Tub 1 or .alpha.-Tub 2 (Carpenter et al., 1990). The maize gene .alpha.-Tub 3 is expressed in all plant organs which have a high content of cellulose undergoing division, in particular in the immature embryos (Montoliu et al., 1990).
It has now been found that the promoter regulatory elements of the regions derived from maize .alpha.-tubulin genes can control a specific tissue expression in the pollen, the radicular systems, the meristematic zones and the immature embryos of plant species, both monocotyledons and dicotyledons. The present invention permits greater control of gene expression in the transgenic plants, permitting especially better control of the herbicide resistance genes.